1. Field of the Invention
The present invention relates generally to gas chromatographic separation and more particularly to an improved instrument oven for gas chromatography separation wherein the air replenishment system permits the oven to operate at near ambient temperatures with minimal temperature gradients.
2. Description of the Prior Art
In the field of gas chromatography separation, sample test apparatus is operated in a temperature controlled oven. A column, usually a long glass tube wound in a circular fashion, packed with coated particles, is a component of the test apparatus suspended in the oven between a sample injector and a sample detector. When a sample is injected, it travels through the column packing until it reaches the detector. Because the time that the sample is in residence or retained within the column packing is indicative of the characteristic identification of the sample, the physical conditions of the column should be repeated for each test run. Therefore, a purpose of the oven is to attempt to provide the same operating temperatures for each test run.
For a gas chromatographic oven to achieve its intended purpose, the oven temperature must be accurately controlled with a minimum of temperature variations. As previously noted, the columns are usually glass and temperature variations will cause the temperature along the columns to vary. Since the sample must be exposed to a homogeneous temperature throughout the column packing to control its retention time and to prevent sample decomposition and other problems, temperature variations or gradients continue to be a persistent problem. Also, there must be a controlled temperature program that is reproducable from one test run to the next to ensure repeatable retention times. The oven must also operate as near to ambient temperature as possible in order to retain samples having low boiling points. This requirement is difficult to satisfy because the injector and detector test apparatus projecting down into the heated oven operate at several hundred degrees centigrade. The oven must also have a large internal volume to accommodate the test apparatus and also be capable of cooling down rapidly from high temperatures to prepare for the next test run.
In the prior art, external air was introduced into the oven when it is operated near ambient temperature to counteract the external heat inputs from the test apparatus. In one attempt, the external air is induced directly into the oven through an opening in the oven door creating high temperature gradients because the cooler ambient air passed directly over the column without any preheating. In a second attempt, the oven door was opened to ambient permitting air to enter and leave randomly around the door edges. This method also leads to high temperature gradients because the cooler air was drawn directly into the oven and the open door served to interface with aisleway traffic. A third attempt utilized a method of premixing the ambient air prior to entering the oven chamber. In particular, one damper controlled the air admitted into the chamber from behind the fan and a second adjacent damper controlled the air expelled from the chamber. Because the two damper openings were adjacent to one another, the flow was unsymmetrical resulting in a short circuiting of the air circulation path. Moreover, two additional openings were required in the oven housing.
Prior to the present invention, Paul Welsh in Hewlett-Packard Company Technical Papers No. GC-67 and GC-68 entitled "The Evaluation and Use of Gas Chromatograph Inspection Ports" and "Evaluation of Commercial Gas Chromatograph Ovens", respectively, explained that high temperature gradients about the test column continued to exist. The result was non-repeatable retention times and sample decomposition such that test runs were difficult to identify and compare.